Electrical musical instrument



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ELECTRICAL MUSICAL INSTRUMENT Filed April 19, 1939 Dec. 31, 1940.

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ATTORNEY Dec. 31, w F CURTIS ELECTRICAL MUS ICAL INSTRUMENT FiledAbril19, 1959 'l sheets-Sheet 7 IN V EN TOR.

WGS) BY d S) ATTORNEY.

Patented Dee. 31, 1940 UNITED STATES PATENT OFFICE ELECTRICAL MUSICALINSTRUMENT Ware Application April 19, 1939, Serial No. 268,786

l2 Claims. (Cl. 84-119) My invention relates broadly to synthetic electrical musical instruments employing a mult*- plicity of generators, andmore particularly to those instruments where each generator originatesseveral tone partials. 'I'he invention is K particularly directed toinstruments in which the generators require a steady exciting orpolarizing potential.

One of the objects of my invention is to provide a circuit employing amultiplicity of generators and wherein a generator is required toproduce several partials simultaneously, the output of the generatorbeing the sum of the outputs taken individually.

Another object of my invention is to provide a circuit arrangement for agenerator having means for applying all pitch and amplitude controls tothe steady exciting potential of a generator, thus avoiding the expenseand the high frequency losses involved in the shielding which is madenecessary by applying these controls to tone-frequency circuits.

Another object of my invention is to facilitate control of attack anddecay rates in an electrical musical instrument.

Still another object of my invention is to provide means for controllingthe variation in harmonic content when the pitch ls changed in afrequency responsive circuit.

A further object of my invention is to provide a circuit arrangement foran electrostaticgenerator system operating without harmonic generatorsabove the range of fundamental frequencies generated and withoutproducing sudden changes in harmonic content.

A still further object of my invention is to provide a circuitarrangement for an electrical musical instrument capable of inexpensiveproduction on a quantity basis for producing a highly satisfactorysynthetic instrument comprising a minimumnumber of operating parts.

Still another object of my invention is to provide a circuit arrangementfor an electrical musical instrument employing a multiplicity of playingkeys and in which the timbre is independent of the number of playingkeys struck.

Another object of my invention is to provide a circuit arrangement foran electrical musical instrument in which the cumulative effects of amultiplicity of generators is obtained when striking a multiplicity ofkeys simultaneously.

A further object of my invention is to provide a circuit arrangement fora synthetic electrical .Y musical instrument'having additive circuitsfor "cumulative association with the same' generator whereby amplitudesof the tones are integrated 'according to the number of keys struckwhile th quality of the tones remains uniform. n Other and furtherobjects of my invention reside in the production of an additive harmoniccontrol circuit for synthetic organs `as set forth more fully in thespecification hereinafter following by reference to the accompanyingdrawings in which:

Figure 1 schematically illustrates the control circuit of my inventionin association with a single generator; Fig. 2 shows the equivalentcircuit of the generator when producing a signal partial; Fig. 3 showsthe equivalent circuit when several partials of different amplitudes areproduced simultaneously; Figs. 4, 4a and 4b conjointly show theschematic circuit connections for a two manualelectrostaticorganembodying the principles of my invention; Figs. 5 and 5a conjointlyshow schematic circuit connections for an electric organ embodying myinvention and using only one contact for each playing key; and Fig. 6shows a modified form of quality control for electric organsillustrating an arrangement of wave-filters in the amplier circuit andwithout the individually connected tappet controlled switches. y

My invention is directed particularly to an improved circuit arrangementfor synthetic electric organ's of the type which employs a multiplicityof electric alternators for generating the notes of a musical scale andtheir associated partial tones. It can be applied to the timbrecontrolarrangements of any one of several of such instruments previously knownto the art. When properly applied to such an instrument, it will resultin improved performance and in considerable simplification.

Instruments of this class have great exibility and are capable ofproducing an almost limitless variety of musical effects. These greatpossibilities are not obtained without price; such instruments areinherently complicated and of necessity contain large numbers ofelectrical circuits. It is inevitable that preventing undesirablereactions among these circuits should be a diiiicult problem. Some ofthe pioneer workers in this field have made little attempt to solve it,merely indicating schematically how the partial tones are to be obtainedand where applied, and,

quite justiably. leaving it to later inventors to work out controlschemes whereby such reactions can be eliminated. Others have arrived atpartial solutions, often at almost prohibitory expense in the way ofauxiliary apparatus. My

invention aims at a complete solution of the reaction problem, togetherwith a substantial reduction in the number of parts required, andwithout loss of useful ilexibility. It also aims at a musicallysatisfactory, controlled, and progressive change in timbre with pitch.Other aims will become apparent as the explanation proceeds.

Some of the requirements which must be iullled vif the instrument is tobe a Worthy competitor of the pipe organ are as follows: (a) When anypossible chord is struck, the timbre must not be noticeably .differentfrom the timbre of a single note. (b) With the volume control left xed,the volume emitted should be approximately proportional to the number ofnotes sounding simultaneously. (c) 'I'he timbre must not change abruptlyin going from one note to another.

Requirements (a) and (b) are both precisely fullled if, when a chord isstruck, the amplitude of each component is equal to the sum of theamplitudes ofthe like partials of the different notes composing thechord, when each is struck singly. By like partials I mean to conveypartials of different notes which agree in frequency. For example, thethird harmonic of any C on the organ, the fourth harmonic of the G belowthe C, the second harmonic of the G above the C, and the fundamental ofthe G which is an octave higher yet, are all like partials as they havethe same frequency. Any system in which the amplitude of each componentof a chord is equal to the sum of the like partials, is hereinaftercalled an additive system. Obviously, it is very desirable to have anadditive system. The diiliculty with requirement (c) generally arisesnear the high frequency end of the instruments range. It lis hard tojustify providing high frequency generators which supply harmonics only,particularly in view of the fact that such harmonics do not exist inpresent acoustical instruments and theoretically seem of little value.However, the dropping oil' of harmonics at the high end of the registermust be obtained without any sudden change of timbre.

It has been proposed heretofore to locate the necessary key switches andharmonics amplitude control elements in the output circuits oi thegenerators. Indeed, Where electromagnetic alternators are employed,-this is the only possible course, as reducing the field current of amagnetic alternator to zero does not reduce the output current to zero,hence control in the input circuit cannot be used. This expedient makesit possible to work out an additive system rather readily, but as theoutput circuit carries audio frequency currents, new difculties areintroduced, to wit: (a)

The large number of extensive circuits tend to couple with each other,capacitively or inductively, and a very complete and hence bulkyexpensive system of shielding must be employed to eliminate spuriousdiscordant notes. (b) This shielding involves losses in stray capacitiesand eddy currents. (c) These losses are diierent or different pitches,thus complicating the voicing" problem. (d) Condenser discharge methodsof regulating attack and decay rates will not work on alternatingcurrents, so that this convenient device cannot be applied directly.

My invention provides an additive system which is applicable to theinput 'circuits of'electrostatic alternators, and he'nce does not incurany of these additional diificulties. It is also applicable to thepolarizing circuits of polarized photocells, "and other manuals.

hence also to photoelectric organs. For simplicity however, only theelectrostatic application has been described. Fig. 1 shows the circuitconnections to a single generator according to my invention. Forpurposes of explaining my invention consider an instrument in which eachgenerator is required to produce n partials, or less, in the case of thegenerators near the end of the fundamental pitch range of theinstrument. These partials may all pertain to a single manual, or somemay belong to one manual and others to In the case of electrostaticgenerators as illustrated herein, the alternating current outputpotential is proportional to the direct current input potential so thatany relationships derived for the input potential of the generators willalso be true of the outputs. Therefore, only the input potential Eg willbe considered. Two cases Will be taken up. In case I al1 the resistorsare equal, and in case II they are considered as unequal, but may alsobe equal, so that case I is actually included under case II. It is takenup separately for simplicity of explanation.

Case I R1=Rz=Rs= :Rn: (say) R From Fig. 2, the generator input potentialwhen a single partial is being generated is where Ex is the potential towhich potential-adjuster Vx corresponding to the given partial isadjusted.

From Fig. 3, the generator potential when a number of partials ofdifferent amplitudes are being generated simultaneously, is

E"- n n n TL The generator voltages for these partials taken one at atime are El, 7L 7L etc., as was shown above. The additive condition istherefore satised.

A numerical example of case I might be helpful at this point. Supposethat two manuals were desired, each utilizing six partials. Then if thetotal polarizing potential available were 480 volts lthe maximumpotential available for any one partial would be 480+12 or 40 volts.While this seems rather small in the light of present v(il) practice, itmust be remembered that my system permits a great reduction in the'capacitances in shunt with the generator outputs, and hence thegenerator output will be a larger fraction of the polarizing potential.If trouble is experienced in obtaining a sufiicient number of millivoltsoutput to blanket the noise due to the connection to the rotors and theinternal amplifier noise, the generators can be divided into groups,each group supplying its own input tube. The shunt capacitance wouldthus be reduced still more. 'Ihe input tubes could all work into acommon amplifier.4

It will be noted later on that some help in this problem can be obtainedby properly adjusting the resistors.

Case II Where R'=el iective resistance of the other n-l resistors inparallel Therefore Similarly,

35 Let these be fundamentals for the two manuals.

Maximum polarizing potential for these partials =1822= 40 volts Thesecould be 2nd, 3rd and 4th harmonics for the two manuals-six-partials inall.

EW: E9 Xvz-l Maximum polarizing potential for these partiels `=2-849=20volts These could be 5th and 6th harmonics forthe two manuals.

Dropping the numerical example again temporarily, we proceed to provethat the system is additive. From Fig. 3

where im=current in any resistor Rm not con nected to zero potential. imis positive if Em Eg and negative if Em Eg.

An equation like this can be set up for each .resistor connectedto-potential above ground.

2"k,E, k,E, E= ,Ek v 2k 2 Ek The additive'condition is then satised.

It may be noted that the foregoing proofs are only valid when at least oe resistor is con# nected to ground potential as otherwisey theequations involve a fraction with zero denominator. It is, however, easyto showby the usey o! the theory or limits that the additive conditionstill holds when all resistors are connected to other potentials.Suppose we add a resistor which is to be always connected from thegenerator input terminal to ground.` Let this resistor be and do notconsider it in the summations. Then finite-1.,* el, disconnectedentirely.

The numerical example already given under vCase-*II may be accepted asillustrative of what may be accomplished by graduating the resistorsg`used as fifth or sixth harmonic, one twentyfourth of the bus potentialis applied to the generator. These fractions will have to be taken intoaccount when the various timbres are setl up. The apparent loss ofvoltage is due to the fact that reserve potential must be available forraising the potential further and further as more and more partials areadded. In fact, a little thought is suilicient to show the truth of the,following, which might be'called first law of additive system. The totalpotential available must be equal to the sum of the maximum potentialsof all partials.

The generators near the ends of the ranges supply a less number ofharmonics than those near the middle, and these have to be given specialconsideration. The fact that it is rather easy to solve the problemswhich naturally arise in a synthetic instrument from this ,cause is oneof the advantages of my system. The generators of the lowest octave areused as fundamentals only, those of the next lowest octave forfundamentals and second harmonics only, those of the lower half of thethird octave for fundamentals, second and third harmonics only, etc. Itwill be found that if a dummy resistor, of resistance equal to R/16, beconnected from the input terminal of each generator in the lowest octaveto ground, the applied voltage for each of these generators will 1"? thesame as that on all other fundamentals. However, as the ear is much lesssensitive to the very low frequencies, it will probably prove advisableto increase the resistance of these dummy resistors in order tofacilitate equalizing the loudness.

At the other end of the scale, it would be desirable to get alongwithout any generators which are used as harmonics only. But if this isdone without making proper compensation, when a timbre rich in highharmonics is being used, an abrupt change of quality and volume will belikely to occur near the top of the scale. Two modiilcations arepossible under the system of my invention. One is the substitutionmethod which has been previously proposedi. e. when a harmonic turns upmissing, a switch and a resistor are provided for it just as though thegenerator were present. The generator end of this resistor is connectedto the input terminal of the fundamental generator, thus substituting anequal amount of fundamental for the missing harmonic. The quality willthus change by steps, but the effective value of the output will remainunchanged regardless of the wave form. My method is to gradually reducethe constants of the harmonic resistors as the upper end of the range isreached. 'I'he quality can thus be made to become simpler gradually, asit does in acoustic instruments.

If the keying resistors are large with respect to the internalresistance of the source, there will be no mutual interference betweenthe input circuits of diierent generators, or the various branchcircuits of the same generator, the resistance of the high potentialportion of any voltage divider being included in the internal resistanceof the source of potential. This fact makes combining the individualgenerators into a complete instrument relatively easy- It is merelynecessary to equip the various switches with mechanical linkages to theproper keys, and connect the switch contacts to the appropriatepotential busses. The busses, ot course, are equipped with means foradjusting their potential.

There are two general methods for connecting the switch contacts. Onemethod consists in connecting all the live switch contacts whichappertain to the same harmonic to the same potential bus. This system isillustrated in the upper manual of the circuit of Figs. 4, 4a and 4b.The other system, which will be explained in due course, is illustratedin Fig. 5. A compromise method for connecting the switch contacts isillustrated in the lower manual of Figs. 4, 4a, and 4b.

Another difculty with present instruments arises from the fact thatsubstantially all of the truly synthetic instruments employ a separatekey-contactor and amplitude control circuit for each partial of eachnote employed such as represented in Patent 1,956,350 to LaurensHammond, April 24,1934; Patent 2,055,719 to Raymond C. Fisher, September29, 1936; and Patent 1,749,685 to Raymond Zouckermann, March 4, 1930.While a considerable change in harmonics content may not be appreciableto the ear, a large number of harmonics are determining factors in thetimbres of various instruments. On this account, it is diflicult for thedesigners of present types of instruments to provide for a suiicientnumber of harmonics 'with a reasonable number of parts. Having providedfor as many harmonics as are practical, they iind themselves confrontedwith a degree of complexity which permits much finer control of harmoniccontent than there is any use for, and at the same time a number ofharmonics that is not as great as desired.

I alleviate this difficulty by dividing the harmonies into relatedgroups, each group to be controlled by a single keycontactor- 'I'heamplitude relations between the members of a group are xed for eachinstrument, by adjusting the relative values of the resistors analogousto R1, R2, R3, R4, and R5 of Fig. 1. The relative amplitudes Within thegroup are chosen by experiment, using the known harmonic contents of alarge number of instruments as a guide. The controls available to theoperator control the amplitudes of entire groups, and thus have a muchgreater eiect on the timbre than controls which affect a singleharmonic. The groups could be selected according to the followingprinciples, or in any other way that seemed desirable to the designer ofa specific instrument.

Harmonics which bear an octave relationship to each other affect thetimbre similarly. Further, it is apparent that harmonics whichcorrespond to notes which form a simple pleasing chord with thefundamental should bear some similarity in their eiTects on the timbre,though the similarity should not be as great as in the oase of theoctave relationship. And nally, harmonies corresponding to notes whichare dissonant to the fundamental would be quite otherwise, having animportant and rather individual effect, probably tending towardsharshness in the iinal quality, but extremely useful musically if onlyfor variety. This feature is illustrated in Manual I of Figs. 4, 4a and4b.

Another feature of acoustical musical instrupass filters in theamplifier.

. filter method is preferable.

ments is the fact that the harmonic content is different for dierentpitches. This has often been considered a defect; on the contrary it ismore likely that it is an essential requirement of an instrument whichis to be musically satisfying for general use over a long period oftime. Itis a well-known fact that a change in harmonic content is moreobvious to the ear than the mere presence or absence of certaincomponents, so that variation in wave-form with pitch is iin-` portantin producing variety. 'I'his effect is the reason for the musicalusefulness of such devices as repeating a passage an octave higher orlower.

Certain musical tones are chiefly characterized by a dependence of theamplitudes of their components on frequency rather lthan on the harmonicorder. Typical tones of this class are the tones of the human voice, theviolin and similar stringed instruments, and the tones of the woodwindinstruments. These tones are all characterized b the existence ofcertain bands of frequenci s, called formants. Regardless of the pitchof the fundamental of the note emitted the high-amplitude componentsalways lie within a formant band. An exception of course exists when thefundamental is above the highest formant; in this case the note isinvariably low in harmonic content.

My system lends itself very readily to quality control by this system.For example, in Figs. 4-40-422 the resistive leads 20l-206 tothepolarizing plates I3 could be led back to a com-mon potential. Thiswould permit the use of a singlecontact key for however many partialswere used. The harmonic content of the originally-generated wave wouldthen be fixed by the values of the individual resistors, which would beseldom, if ever, changed. Quality control would be effected ordinarilyby one of two alternative systems. One method would be to use a set ofmultiple band- These filters would be interchangeable by means ofquality-control switches operable from the console. or the same filterwould be modified by said switches; in which cases the filter elements(inductances, capacities, and resistances) would be adjustable by meansof tap-switches or otherwise. Another method would be to connect theoutput plates l5 of the individual generators, singly or in groups ofadjacent pitches, to the vamplifier through individual attenuators. Thesecond method would perhaps give more positive control of the quality,but would be more expensive and more complex. In the present state of.the art, the characteristics obtainable by filters are almostunlimited, particularly if regenerative vacuum tube circuits orelectro-mechanical filter devices are employed. Taken by and large,then, it would seem that the Fig. 5 shows the connections for thekey-switches and gener tors when using this system. Fig. 5a shows theassociated Itimbre-control.circuits using individual attenuators; Fig. 6shows an alternative timbrecontrol scheme using interchangeable filters.

This system, which may be called a pure formant system of qualitycontrol, is very attractive from the viewpoint of simplicity ofconstruction, but has certain drawbacks. In the first place, thegenerators used in this manner cannot be employed in the ordinaryharmonic-order control circuit. That is, while the generators andIth'e'basic generating circuit'is the same in both cases, thequality-control circuits are so different .that vthe .two systemsdescribed cannot be employed alternatively inthe same instrument.

at least not without an almost prohibitively complex switching system.In the second place, it will be noted that the acoustic instrumentsfunctioning on the formant system are solo instruments, in the sensethat only one note at a time is produced. (In the case of the violin,two notes can be sounded simultaneously, but they will be close togetherin pitch.) Thus, the volume discrepancies that might otherwise arise'are taken care of by the operator, note by note. In the case of akeyboard instrument, where several notes remote from each other in pitchare often sounded simultaneously, a difliculty would arise. It is notknown at present how serious this difficulty would be. In any case, thediiiiculty can be overcome by using a volume limiting tube such aselement 302 of Figs. 5a and 6r and explained below.

I have devised a circuit which eliminates these diiculties at theexpensel of employing a keycontrolled switch for each harmonic. Thiscircuit is similar to the circuit previously described, except that thevoltage tap to which the individual resistors are connected depends uponthe pitch (frequency) of the generator instead of the order of harmonicas in the previous case. This system consists in dividing the generatorsinto groups, the generators in each group comprising a series ofadjacent pitches. All live switch contacts in the group, regardless ofharmonic number,` are connected to the same potential bus. A circuit ofthis type is illustrated in manual II of Fig. 4a. This may be called acompromise or semi-formant system.

In Fig. 1 the electrostatic generator assembly is indicated at G and issimilar to the structure set forth in my Letters Patent 2,001,708, datedMay 2l, 1935. The control contactors are indicated at Ki, K2, K3, K4,and K5 having sets of opposite coacting contacts as shown. The controlcontactors Ki, K2, K3, K4, and K5, etc., are each operated by adifferent key. The means for coordinating .the movement of thecontactors are indicated schematically by dotted lines The connectionsto .the source 2 from the coacting contacts of the contactors areindicated at la, 2a, 3a, 4a, and 5a. Reference character 3 designates avolume determining resistor between the generator G and the input of theelectron tube amplifier 4. Quasi-linear properties may be imparted togenerator G through coupling circuit constituted by condenser C andresistances 3 and 5 whereby the impedance of the generator and the inputof the 4amplifier are properly related for obtaining proportionalitybetween active plate area and Iamplifier input potential, as explainedin my Patent 2,001,708. Amplifier 4 has a suitable cathode heatingsupply as indicated at 6 and a suitable plate circuit supply shown at 1.The output .transformer 8 in the output of amplifier 4 connects to asuitable sound reproducer indicated at 9. Condenser I8 produces gradualattack and decay of tones by the method disclosed in Floyd A. FirestonePatent 1,953,753, granted April 3, 1934.

In Fig. 2, I have shown a circuit equivalent to Fig. 1 for a generatorproducing a single Partial according to the mathematical demonstrationsupra. Like reference characters refer to similar parts described inconnection with Fig. 1.

In Fig. 3, I have shown circuits similar to Figs. 1 and 2 using similarreference characters to designate similar parts and showing anequivalent tudes are produced simultaneously.

The circuit connections for a two-manual organ constructed in accordancewith my invention are shown schematically in Figs. 44a-4b. Figs. 4-4aand 4b are continuations of one circuit. The location of parts in thegures is not intended to suggest the location in the actual instrument,but was intended merely to facilitate drawing in the electricalconnections, as is usual in schematic circuit diagrams. No attempt hasbeen made to show all the apparatus in ythe instrument nor all theconnections, as it was felt that the principles to be followed could bemore readily understood if too great repetition of parts and circuitswas avoided.

In Fig. 4, each of elements I3 represents one stator plate of avarying-capacity generator, each of elements I5 the other stator plate,and I4 the intervening rotor plate. The generators have additionaldesignations comprising letters with subscripts. `These show the note ofthe musical scale which the generator emits when used as fundamental.Thus Ci indicated C in the lowest octave, E4, E in the fourth octavecounting from the bottom, etc. Elements I6 represent individual voicingresistors, i. e., resistors for adjusting all notes to the sameloudness. Each output plate I5 connects through a voicing resistor I6and conductor 3| to the grid electrode of amplifier tube l34 in Fig. 4b.

Elements to |I2 and 20| to 206 are the individual harmonic-controlresistors analogous to those shown at Ri-Rs in Figs.,1 to 3. In thesedesignations, the rst digit gives the manual and the last two digits theorder of the harmonic with which the resistor is associated. Thus a |0|resistor would control a fundamental tone of manual I, a 205 resistor afifth harmonic of manual II. etc.

Each of these resistors has one terminal connected to an input plate I3,and the other terminal connected to the movable contact of akey-controlled switch I9. All of-the resistors connected to each of thegenerators shown are represented. As not all of the key-controlledswitches are shown,.the connections to one terminal of many resistors|0I-I I2, 20|-206 had to be omitted. It is to be understood that in theactual instrument each of these terminals is to be connected to themovable contact of the appropriate key-controlled switch.

A plurality of switches I8 are controlled by each playing key. Inaccordance with the convention used in radio for showing a gang-switch,gang-condenser and the like, the mechanical linkages whereby the playingkeys operate these switches are represented by dotted lines. Linkagesdesignated with the numeral I are operated by playing keys of manual I:linkages designated with the numeral 2 are operated by playing keys o1manual II. These linkages are further desighated byletters withsubscripts which identify the playing key which actuates them. 'I'husIC1 indicates the lowest C of the flrst manual, 2G; the lowest G of thesecond manual, etc.

Each of switches I8 has three contacts, a movable contact, and two xedcontacts. The iixed contact which is in use when the key is in thenon-playing position (up) is connected toV ground or zero potentialthrough conductor 30 in all cases.

suitable steady `potential according to the following scheme:

In manual I, control by the harmonic-order 7i-system is exemplified, thefeature vo f multipleharmonics per contac/t/ being incorporated. EachThe other fixed contact connects to a IOI resistor (i. e., eachfundamental) connects through a separate switch I8 to conductor 20,which is supplied with a potential controlled as shown in Fig. 4b, whichwill be hereinafter explained. The second, fourth, and eighth harmonic'sbeing in octave relationship to each other (and the fundamental) arecontrolled by a single switch I9 as follows. Plate I3 of the generatorproducing the 2nd harmonic of the note under consideration is connectedthrough a |02 resistor to the movable contact of a switch I9; plate I3of the fourth harmonic generator is connected to the same point througha |04 resistor, and plate I3 of the eighth harmonic generator to thesame point through a |08 resistor. The active contact of this switch I9is connected to conductor 2|. which is supplied with a controlledpotential by the circuit shown in Fig. 4b. Now the relative amplitudesof the second, fourth, and eighth harmonics are controlled by therelative resistances of resistors |02, |04 and |08. The amplitude of thegroup as a whole is controlled by the potential applied to conductor 2I. The sixteenth harmonic could be added to this group if desired.

Similarly, the third, sixth, and twelfth harmonic generators aresupplied with potential from conductor 22 through a common switch I9.(The frequency of these and succeeding harmonies is of course onlyapproximately correct.) The fifth and tenth harmonic generators aresupplied with potential' from conductor 23 through another common switchI9. The frequencies so far are all involved in the chord known as themajor triad in the study of harmony. 'I'he readily available dissonantharmonics, the seventh and the ninth, are lumped in the last group, andsupplied potential through conductor 24.

It will be noted that this arrangement is in accordance with theprinciples previously stated. Many variations of the arrangement arepossible, some of which may be preferable under certain conditions thanthe arrangement set forth herein and it will be understood that mydisclosure herein is to be considered in the illustrative sense and notas limiting my invention to the specific arrangement shown.

Certain conductors in manual I are shown unterminated, as they lead toapparatus not shown. That is to say the circuits are repetitions of thecircuits heretofore described. Thus conductor |20 goes to plate I3.ofthe D3 generator (not shown) through a |03 resistor (not shown).Similarly, conductor I2| goes to plate I3 of the B3 generator through a|05 resistor, and plate I3 of the B4 generator through a I I0 resistor.Conductor |22 goes to plate I3 of the Fg generator through |01 resistor,and to plate I3 of the A4 generator through a |08 resistor. Conductor|23 goes to plate I3 of the G5 generator through a I|2 resistor.Conductor |24 goes to plate I3 of the E generator through a |I0resistor. Conductor |25 goes to plate I3 of the D5 generator through a|08 resistor. Conductor |28 goes to plate I3 of the Ats generatorthrough a |04 resistor and to plate I3 of the Ata generator through a|88 resistor. Conductor |21 goes to plate I3 of the A35 generatorthrough a |06 resistor and toj plate I3 of the Ate generator through a.||2 resistor. Conductor |28 goes to plate I3 of the M5 generator througha |08 resistor. Conductor |28 goes to plate I3 of the Ate generatorthrough a IIO resistor. Conductor |30 goes to plate I3 of the AIIsgenerator through a |I2 resistor. Conductor |3I goes to plate I3 of theAi: generator through a |02 resistor. Conductor |32 goes to plate I3 ofthe F114 generator through a |01 resistor. Conductor |33 goes to plateI3 of the At: generator through a |03 re- 5 sistor.

Reference character 50 is a three-position multi-gang switch. Whenthrown to the position farthest to the left on contact 53 it associatesa large condenser 5I with resistors |0| to |I0, so as l@ to produce a.very gradual attack and decay, in a manner explained by Firestone (loc.cit.) When switch 50 is placed in the center position on contact 54, asmaller condenser 52 is substituted, giving a less gradual attack. Whenswitch 50 is i3 placed in the position to the extreme right on contact55 the condensers are disconnected and an abrupt attack results.

'Manual II is co'nnected on the compromise or semi-formant system. Asidefrom the number of i2@ harmonic-control resistors and key-controlledassociated with generators in the lower half of :ige third octaveconnect to conductors 42, 43, 44,

etc. In Fig. 4b, 34 is an amplifier tube supplied with energy from thegenerators through conductor 3| land volume control 32. Volume control32 should l be operable from the console, preferably by means of a footpedal. Vacuum tube 34 feeds loudspeaker 36 through an interveningvacuum-tube system typified by tube 35.

40 Numeral |40 designates the mechanical linkage of a. multiple switchwhich may be called an adinstable-quality tappet. When this tappet isdepressed, indiVIdulaJ'sWitches I4| render adjustable potential dividers|42 'eiective in controlling the 45 potential of conductors lli-I4.Potsdam dividers |42 obtain their electrical energy from common sourceof potential 36.

Numerals and |60 designate the mechanical linkages of multiple switcheswhichmay be called 5 fixed-quality rappers. with tappet |40 raised,depressing |50 or |60 forms a potential divider of which the upper legis resistor |52 or |62 and the lower leg is resistor |35. The value ofresistors |52 control the potentials impressed on conduc- 55 tors |0-I4when tappet |50 is depressed. Similarly another tone quality, dependinguponthe values of resistors |62, results from depressing tappet |60.

80 pressed simultaneously, in which case the amplitude of eachindividual harmonic is greater than',

it would be with only one tappet depressed, but not as great as the sumoi' `the individual amplitudes. Tappet |40 can be operatedsimultaneously with one or more fixed-quality tappets, in which case itfunctions as an adjustment on the quality already setup by thefixed-quality tappet. The amplitude of any given harmonic may beincreased or decreased, depending upon the relation between the originalamplitude of the harmonic and the setting of the correspondingadjustable potential divider |42. The magnitude of thiseiect dependsupon the magnitude of resistors |43, being larger when |43 is smaller.

g5 Additional fixed-quality tappets similar to |50 Tappets |50 and |60may bedeand |60 can be'added as desired. A truly additive circuit forthe tappets can readily be worked out along the lines previously givenfor the generators; but as such a circuit would require more polarizingvoltage and would'be more complex 5 to design and construct, it isbelieved that the circuit given is preferable.

Numerals 250 and 260 designate the mechanical linkages for fixed-qualitytappets for the second manual. They function in the same manner 10 asthe tappets for the rst manual. An adjustable-quality tappet could beadded along previous lines, but under the formant system there does notseem to be a great deal of point in doing this.

In Figs. 5 and 5a the circuit connections for 15 the pure formant schemeof quality control, using only one contact per playing key, are shown.Figs. 5-5a are drawn as though there were only three notes per octaveand four octaves in the complete instrument; this is for the sake ofsim- 20 puoityoniy, and it is to be understood that the arrangement maybe readily extended to the regulation twelve semitones per octave and asmany octaves as desired. Again for the sake of simplicity, only threeharmonics are shown. The in- 25 dividual harmonic control resistors are30| for the fundamental, 302 for the second harmonic, and 303 for thefourth harmonic, the third not being used in the illustration. Theseresistors are shown variable, and are ganged together by the 3mechanical linkages 304, 305, and 306 respectively. This constitutes anoptional auxiliary quality control. For reasons of expense, this wouldin practice probably be accomplished by resistors with one or two tapsand a gang switch, as a 35 large number of resistorsvhave to becontrolled if this feature is to be included. The 302 and 303 resistorsare at all times to be graduated so that the resistors for the highestnotes have the highest value of resistance, to avoid sudden changes 4 inquality, as previously explained.

By tracing out the circuit, it will be noted that each of the playingkeys 300 which control notes in the highest octave (last three notes onthe right of the diagram) connects plate I5 oi.' one of 45 thegenerators to potential source 30| through a 30| resistor. 'I'his is theonly connection made in sounding a note of the highest octave, therebeing no harmonic generators above the fundamental range, as previouslyexplained, so that the 5 notes of the highest octave are substantiallypurer sine waves-a condition which is approximated by many instruments.The playing keys of the next highest octave each connect plate I5 of one,Y generator to source 303 through a 30| resistor, 55

and plate I5 of another generator to source 300 through a 302 resistor.The notes in this octave A thereforecontain fundamental and secondharmonic components only.

Each of the stator plates .I3 of the generators is connected through a301 resistor to one of the conductors 3| 0--3I4. Resistors 301-simultaneerators connect to conductor III, the next two to 70 conductor3| etc. Conductors 3|0 to 3|4 connect individually to fixed4quality-control tappet 320 and adjustable quality-control tappet 330.Thence the potentials from the various groups of generators, each nowhaving its appropriate amplitude as set by resistors 32| and/or 33|, isapplied to the input circuit of amplifier tube 340. The output circuitof tube 340 contains transformer 34|, whose secondary is Wound forpushpull operation. Connected to this push-pull secondary is doublediode tube 342, which functions as an automatic volume limiting deviceto prevent sudden burst of sound in case a note having a strong partialin a band which is little attenuated by resistors 32| or 33| is sounded.The cathode of double diode 342 is given an adjustable positive biasthrough resistor 348. As long as the audio frequency potential acrossthe secondary winding of transformer 34| is insuillclent to raise thepotential of the diode plates above the potential of the cathode at anytime, the diode remains inoperative. But if the peaks of the audiofrequency potential are sufiiciently large, the diode will draw current,thus shunting the transformer secondary and reducing its efiiciency as acoupling device.

The audio frequency potential, as limited by this device, is thencetransmitted to volume control 343, which is operated manually or by afoot pedal. transmitted to loud-speaker 341.

Figure 6 gives an alternative method of quality control. Conductors 3|0-3|4, instead of being connected individually to tappet-controlledswitches, are all connected together to the amplier input circuit. Theamplifier is then'provided with a number of wave-filters, arranged to beemployed alternatively. In Fig. 6, a gangswitch comprising individualmulti-point switches 39|ia and 390D mechanically linked together, isemployed for selecting the wave-filters. Three different types of-filters have been shown. No importance is to be attached to the precisecircuits of these filters; they are intended to be typical of thenumberless circuits known to the art which will provide band-pass ormultiple band-pass characteristics. The network 360 is a fairly commontype ol band-pass filter employing three inductances and threecapacities. Voltage divider 352, amplifier 350, regeneration-controlvario-coupler 35|, tuned circuits 353, choke coil 354 and blockingcondenser 355 comprise a filter with band-pass characteristics sharpenedby regenerative means. 'I'he third interchangeable filter is amagneto-striction type adapted from a filter circuit employingpiezo-electric elements. Magnetostrictive rod 380 is wound with a(multilayer) helical coll 38| having center tap 382 connected throughcondenser 333 and tuned-circuit 384 back to ground.

I have no intention of making any claims on these filters per se,neither as to originality nor as to the operability of the exact circuitshown.

I do, however, maintain that filter circuits designed along the generallines shown are Well known in the art and can be made to fulfill a widevariety of frequency-response conditions; and that such iilters, used incombination with generating and keying circuits substantially as heredisclosed, constitute a new and useful device for producing music.

While I have described my invention, therefore, in certain preferredembodiments, I intend no limitations thereby upon my invention; and Idesire it to be understood that modifications may be made and variousdevices other than those specified may be employed within the scope ofthe appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is as follows:

After further amplification, it is then' 1. In an electrical musicalinstrument including a multiplicity of playing keys controlling amultiplicity of electric alternators and an electric sound producercontrolled by said alternators, the combination of a pair of inputterminals for each alternator, one or more switches operated by each ofsaid playing keys, each of said switches comprising a pair of xedcontacts and a movable contact controlled by means of said keys,electrical circuit connections including one of said input terminalsfrom each of said alternators and one of said fixed contacts from eachof said switches connected to a common point, a source of potentialconnected to said common point, a plurality of conductors connected tosaid source of potential through adjustable potential dividers, aconnection from each remaining fixed contact of said switches to someone of said conductors, and resistive connections from each movablecontact of said switches to one or more of the remaining input terminalsof said alternators such that the timbre changes progressively withpitch.

2. In an electrical musical instrument including a multiplicity ofplaying keys controlling a multiplicity of electric alternators and anelectric sound producer controlled by said alternators, the combinationof a pair of input terminals for each alternator, one or more switchesoperated by each of said playing keys, each of said switches comprisinga pair of fixed contacts and a Inovable contact controlled by means ofsaid keys, electrical circuit connections including one of said inputterminals from each of said alternators and one of said fixed contactsfrom each of said switches connected to a common point, a source ofpotential connected to said common point, a plurality of conductorsconnected to said source of potential through adjustable potentialdividers, a connection from each remaining fixed contact of saidswitches to some one of said conductors, and resistive connections fromeach movable contact of said switches to one or more of the remaininginput terminals of said alternators, the contacts of said switches whichare connected to any one of. said conductors all being associated withthe same alternator or with alternators productive of adjacent pitches.

3. In an electrical musical instrument, the

combination as set forth in claim 1, with means for selecting thepotential conductor to which certain of said remaining fixed contactsare connected on the'basis of pitch, and means for selecting thepotential conductor to which the balance oi' said remaining ilxgdcontacts are connected on the basis of harmonic order.

v 4. In an electrical musical instrument including a plurality ofalternators productive of different musical frequencies, and a soundreproducer, the combination of input and output circuits for saidalternators, key-controlled contactors in said input circuits forrendering said alternators effective upon said sound reproducer, andresistive electrical connections from each key-controlled contactor tocertain selected alternators, the frequencies of said selectedalternators being so related that when each of the corresponding notesis sounded simultaneously with the lowest-pitched of them, the resultingchords are simple ones which are similar to each other or at leastclosely related to each other, the simplicity and the relatedness ofsaid chords being decided according to the teachings of the musical artof harmony.

5. In an electrical musical instrument includcaemos 9 ing a multiplicityof contactors operated by play' ing keys, each key controlling a singlecontactor, a multiplicity of alternators having input and outputterminals, a source of potential, and an 'electrical sound-producingdevice'the combination of electrical connections between the inputterminals ofvsaid alternators, said contactors and said source ofpotential for supplying the input terminals of more than one of saidalternators with electrical energy from said source of poten- -tial whenone of said playing keys is depressed,

and a plurality of amplitude control circuits connected to said inputterminals of said alternators, said control circuits being so arrangedthat the resultant amplitude of any one component of a musical note is apredetermined function of its Y pitch? 6. In an electrical musicalinstrument includv ing a multiplicity of contactors operated by playingkeys, a multiplicity of alternators having input and output terminals, asource of potential, and an electrical sound-producing device thecombination of electrical connections between the input terminals ofsaid alternators, said contactors and said source of potential forsupplying the input yterminals of more than one of said the magnitude ofthe resistances of the indi input and output terminals, a source ofpoten,

tial, and an electrical sound-producing device the combination ofelectrical connections be-A tween the input terminals of saidalternators, said contactors and said source of potential for supplyingthe input terminals of more than one of said alternators with electricalenergy from said source of potential when one of said playing keys isdepressed, a plurality of amplitude control circuits connected to saidinput terminals of said alternators, each of said control circuits beingindividual to one frequency or common to a band of adjacent frequencies,and means for variably adjusting each of said control circuits.

8. -In an electrical usical instrument, the circuit connections asefined by claim 1, with vidual resistive connections therein describedbe-` ing graduated so that the relative amplitudesf output of thepartial tones will change from note to note in a musically pleasingmanner.

9; In an electrical musical instrument including a plurality of electricalternators operating at different frequencies, an electric soundproducer and playing keys for rendering frequencies produced bythealternators effective in the sound producer, in combination with inputand output terminals for said alternators, a source of potentialconnected -in common with one input terminal of each alternator,.andindividual means energized at diiferent potentials from said source andconnected with selected alternators at the remaining input terminalsthereof for providing tone partials of different amplitudes at thevarious frequencies; the output terminals of said alternators beingconnected in common with the electric sound producer, each of theplaying keys being arranged for sounding selected partialssimultaneously in the sound producer. and the total potential availableat said output terminals being substantially equal to the sum of themaximum potentials of all the partials sounded, regardless of the numberof playing keys operated.

10. In an electrical musical instrument, the combination set forth inclaim 9 wherein said individual means comprise resistors of equal valuewhereby the amplitudes of the partials produced are proportional to thedifferent potening a plurality of eleptricalternators operating' atdifferent frequencies, an electric sound' producer and playing keys forrendering frequencies produced by`the alternators effective in the soundproducer, in combination with input and output terminals for saidalternators, a 'source o'f 'pow' tential connected incommon with oneinput,4 *t terminal z of ,each alternator, andindividual means connectedfrom said source to selected alternators vat the maining' input tethereof for providing tone partials of different amplitudes at thevarious frequencies; the outv ducer, with the tota1 potential availableatsa1dterminals being substantially equal to,` the sinn of themaximumpotentials of all the partials sounded,` l e

